What performance indicators will be changed by the stretching orientation process of unidirectional
Unidirectional polyimide film is stretched in a single direction before and after imidization, causing the polymer chains to be oriented along the stretching direction, resulting in significant anisotropy in the longitudinal and transverse properties of the film, and changing various performance indicators below.
Unidirectional polyimide film is stretched in a single direction before and after imidization, causing the polymer chains to be oriented along the stretching direction, resulting in significant anisotropy in the longitudinal and transverse properties of the film, and changing various performance indicators below.
1、 Mechanical performance indicators
1. Tensile strength and elastic modulus: The orientation of molecular chains in the tensile direction (longitudinal) significantly increases the longitudinal tensile strength and elastic modulus; Untretched transverse molecular chains are disordered, with limited improvement in transverse tensile strength and modulus, and a widening gap in longitudinal and transverse mechanics.
2. Fracture elongation: The longitudinal fracture elongation decreases, and the film becomes brittle along the stretching direction; The lateral elongation is relatively higher. Excessive orientation can lead to brittle fracture in the longitudinal direction.
3. Tear resistance: The longitudinal tear resistance decreases, and cracks are prone to propagate along the orientation direction, resulting in longitudinal splitting; Lateral tear resistance is relatively good.
2、 Thermal dimensional stability index (thermal shrinkage rate)
After stretching orientation, there is internal stress in the film. Under high temperature conditions, significant thermal shrinkage will occur along the longitudinal direction of tension; Horizontally without orientation, the thermal shrinkage value is very small. The higher the degree of orientation, the greater the longitudinal thermal shrinkage rate; If the heat setting process is insufficient, the shrinkage phenomenon will become more prominent during high temperature use.
3、 Thermomechanical related properties
1. Thermal expansion coefficient: The longitudinal thermal expansion coefficient decreases; The lateral thermal expansion coefficient does not change significantly, and the longitudinal and transverse thermal expansion coefficients of the film are not consistent. During downstream composite and overlay processing, warping and wrinkling are prone to occur.
2. Glass transition temperature Tg: Moderate orientation will slightly increase Tg; excessive orientation and accumulated internal stress will exacerbate the deformation caused by stress release at high temperatures.
4、 Dielectric insulation performance
1. The directional arrangement of molecular chains will change the density of molecular stacking inside the film, and the dielectric constant will also change;
2. Breakdown voltage: Moderate stretching increases density and improves breakdown strength; Excessive stretching can cause microcracks and small defects, leading to a decrease in breakdown voltage;
3. Dielectric loss: Improper orientation process control generates internal residual stress, and dielectric loss will increase under high temperature conditions.
5、 Physical appearance and processing performance
1. Transmittance: Improper control of stretching orientation can result in crystal dots, micropores, increased film haze, and decreased transmittance performance.
2. Shrinkage curling tendency: Residual oriented internal stress makes the film prone to unidirectional curling after storage and heating.
3. Cutting performance: It is easy to split longitudinally, and burrs and longitudinal micro cracks are prone to occur during cutting.
6、 Chemical and solvent resistance performance
The molecular chains are orderly and tightly stacked, and the resistance to solvent erosion in the stretching direction has been improved; But the horizontal molecular arrangement is loose, and the solvent resistance is weaker than that of the vertical direction; The film exhibits chemical resistance anisotropy.
7、 Breathable and Permeable Performance
The molecular chains are tightly arranged along the stretching direction, and the diffusion resistance of gas and water molecules increases in the longitudinal direction; The lateral diffusion coefficient is relatively higher, and the permeability of the film varies in all directions.
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